High-strength feldspathic dental porcelains were first introduced in the dental industry in the 1980s. Optec™ porcelain, the subject of U.S. Pat. No. 4,798,536, is one example of feldspathic porcelain that has been used in the manufacture of all-ceramic dental restorations. Currently, the most common technique for manufacturing all-ceramic dental restorations is heat-pressing, also known as injection molding, of all-ceramic cores. Dental glass-ceramic materials, such as OPC® pressable ceramic and Empress® pressable ceramic, exhibit flexure strength from about 120 MPa to about 180 MPa due to a relatively high fraction of leucite crystals embedded in the glass matrix. Historically, the first leucite-containing components of dental porcelains were produced by melting potassium feldspar with fluxes as described in U.S. Pat. No. 4,798,536, which is hereby incorporated by reference. Thus, these glass-ceramic materials are often also referred to as high-strength feldspathic dental porcelains or leucite-reinforced dental ceramics or glass-ceramics. The leucite content in these materials is higher than at least about 35 weight percent and most of the porcelains have a leucite content from about 45 to about 60 weight percent. As a result, they exhibit relatively high coefficients of thermal expansion (CTE) in excess of about 15×10−6/° C. (from room temperature to 500° C.) and require specifically developed porcelains with a matching high thermal expansion. At the same time, most of the porcelains used for porcelain-fused-to-metal (PFM) restorations have CTEs of about 12 to about 13×10−6/° C. (from room temperature to 500° C.) and are compatible with most commonly used precious and non-precious alloys having CTEs in the range of about 13 to about 15×10−6/° C. (from room temperature to 500° C.). Thus, these porcelains are commonly referred to as conventional porcelains.
One of the distinct advantages of the heat-pressing (injection molding) method for fabricating all-ceramic cores is exceptionally accurate and consistent margin adaptation. Only very few and the most experienced dental technicians can achieve the same accuracy building porcelain margins by hand. The most time-consuming procedure is manual fabrication of porcelain margins on multi-unit metal frameworks for fixed partial dentures (FPD) or bridges. To circumvent these difficulties, a new method commonly referred to as “pressing to metal” was recently introduced, where a heat-pressing technique is used to press high expansion leucite-based glass-ceramic directly onto a metal framework. As a result, to produce the finished restoration, the technique requires layering the press-to-metal core with the porcelain having a coefficient of thermal expansion exceeding that of conventional porcelains.
It would be extremely beneficial to have pressable all-ceramic cores compatible with these aforementioned porcelains and at the same time capable of being directly pressed to metal.